A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) of a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Photolithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. At present, no alternative technology seems to provide the desired pattern architecture with similar accuracy, speed, and economic productivity. However, as the dimensions of features made using photolithography become smaller, photolithography is becoming one of the most, if not the most, critical gating factors for enabling miniature IC or other devices and/or structures to be manufactured on a truly massive scale.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size of a feature arranged in an array with a 1:1 duty cycle (i.e., equal lines and spaces or holes with size equal to half the pitch). It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to improve resolution performance of a lithographic system, various tools may be used. In one approach, an illumination system of a lithographic apparatus is refined by considering alternatives to full circular illumination shapes. Such full circular illumination shapes are also referred to as conventional illumination. A radiation system of a lithographic apparatus generally includes an illumination system. The illumination system receives radiation from a source, such as a laser, and produces an illumination beam for illuminating an object, such as the patterning device (e.g. a mask on a mask table). Within a typical illumination system, the beam is shaped and controlled such that at a pupil plane of the illumination system the beam has a desired spatial intensity distribution. Such a spatial intensity distribution at the pupil plane effectively acts as a virtual radiation source for producing the illumination beam. Various shapes of the intensity distribution, consisting of (substantially uniform) light areas on a dark background, can be used. Any such shape will be referred to, hereinafter, as an illumination shape, an illumination mode, an illumination configuration, an illumination setting or a shape of an illumination source. A maximum selectable extent of aforementioned virtual radiation source is defined by the design of the illumination system (e.g., the optical extent of the illumination pupil), and corresponds to a maximum clear aperture size in the illumination pupil.
A system where illumination radiation is obliquely incident on the patterning device at an angle so that the zero-th and first diffraction orders are distributed on alternative sides of the optical axis may allow for improvements. Such an approach is generally referred to as off-axis illumination. Off-axis illumination improves resolution by illuminating the patterning device with radiation that is at an angle to the optical axis of the projection system. Examples of off-axis illumination include multipole illumination and annular illumination. The incidence of the radiation on the patterning device, which acts as a diffraction grating, improves the contrast of the image by transmitting more of the diffracted orders through the projection system. Off-axis illumination techniques used with conventional masks produce resolution enhancement effects similar to resolution enhancement effects obtained with phase shifting masks. Besides off-axis illumination, other currently available RET include optical proximity correction (OPC) of optical proximity errors (OPE), and sub-resolution assist features (SRAF). Each technique may be used alone, or in combination with other techniques to enhance the resolution of the lithographic projection tool.
As illumination systems have evolved from producing conventional to annular, and on to quadrupole and more complicated illumination configurations, the control parameters have concurrently become more numerous. In a conventional illumination mode, a circular area including the optical axis is illuminated in a pupil of the illumination system, the only adjustment to the illumination mode being to alter the outer radius (σr) of the circular illumination shape. Annular illumination requires the definition of an inner radius (σc) in order to define the illuminated ring of the annular illumination mode. For multipole patterns, the number of parameters which can be controlled continues to increase. For example, in a quadrupole illumination configuration, in addition to the two radii, a pole angle α defines the angle subtended by each pole between the selected inner and outer radii.
Concurrently, patterning device (e.g., mask) technology has been evolving as well. Binary intensity masks have given way to phase shift masks and other advanced designs. While a binary mask simply transmits, reflects or blocks imaging radiation at a given point, a phase shift mask may attenuate some radiation or it may transmit or reflect the light after imparting a phase shift, or both. Phase shift masks have been used in order to image features which are on the order of the imaging radiation's wavelength or smaller, since diffraction effects at these resolutions can cause poor contrast and end-of-line errors, among other problems.
Modern illumination systems have ever increasing numbers of variables which can be manipulated. In order to account for the various permutations of variable settings and to reduce the cost of trial and error optimization of illumination configurations, photolithographic simulations may be used to optimize the illumination conditions for a given mask pattern.
One approach for determining an optimal combination of the illumination shape and the patterning device pattern (e.g., the mask pattern) is to calculate the normalized aerial image log slope (NILS) at a number of pre-selected points, commonly referred to as fragmentation points, along the border of pattern features. Then, the intensity and shape of the illumination and the magnitude and phase of the diffraction orders from the patterning device pattern are simultaneously changed to form an image in the image plane that maximizes the minimum image log slope at the fragmentation points while forcing the intensity at the fragmentation points to be within a predetermined intensity range.
While maximizing NILS at selected sampling locations in the pattern enhances the budget/tolerance for exposure variation, commonly referred to as the exposure latitude EL, it may not help to increase the budget/tolerance for focus variations, commonly referred to as the depth of focus (DOF). Furthermore, results obtained with this approach generally may suffer at low k1, where pure image calculations deviate substantially from printed substrate results.